I'm trying to write my own high quality audio sample rate converter. I barely know anything about signal processing though so I need help. From what I understand I need to sum together normalized sinc functions that touch every sample to find the value at any arbitrary point. I'm guessing this is hard because that would mean more than a billion calculations for a few second long audio file. So do I only use the sinc functions for the samples that are closest to my x value? Is this what is meant by "windowed sinc"? How many samples should I go in each direction away from my x value? Additionally, my DAW has something called "32 point sinc" resampling. What is the "32 point" supposed to mean in this case?
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1$\begingroup$ Have a look at dsp.stackexchange.com/questions/59740/… and the comments on dsp.stackexchange.com/questions/69033/… $\endgroup$– Cedron DawgJul 13 '20 at 2:10
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$\begingroup$ @CedronDawg I think that it is worth putting those in an answer. This question is "far enough" from those original topics to be deemed a duplicate. $\endgroup$– A_AJul 13 '20 at 13:48
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$\begingroup$ @A_A I'm not understanding what you mean. The referenced questions are meant to give an understanding of how a sinc interpolation (for upsampling) works and how it relates to maybe using a FFT for efficiency. So, they don't really answer the OP directly, but give context. The analysis of error consequences vs truncation length is a different beast and I would defer to Olli as he does a spectacular job on those. $\endgroup$– Cedron DawgJul 13 '20 at 15:34
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1$\begingroup$ @CedronDawg No I certainly do not mean close the question just like that. If the OP accepts it, then the question is not going to remain "unanswered" and keep coming back. $\endgroup$– A_AJul 15 '20 at 12:05
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1$\begingroup$ @olliniemitalo The "32 point sinc" is a option in FL Studio. $\endgroup$– sincmanJul 18 '20 at 9:04
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"So do I only use the sinc functions for the samples that are closest to my x value?"
Yes, when you are truncating.
"Is this what is meant by "windowed sinc"?"
Yes. The sinc goes to infinity, calculating that is impractical.
"How many samples should I go in each direction away from my x value?"
The sinc diminishes the further away you are. At some point additional points become insignificant (no wider than a whisker on a gnat).
"Additionally, my DAW has something called "32 point sinc" resampling. What is the "32 point" supposed to mean in this case?"
I'm pretty sure it means a 32 point window, centered on your current sample.
Sinc interpolation is one form of interpolation. It is the "Fourier compatible" one as it gives limits on a bandwidth basis.
Here is another link that may be even more useful: Multi-channel audio upsampling interpolation
On re-reading this reference, Olli has already done this analysis, spectacularly.
answered Jul 13 '20 at 16:11